1. Field of the Invention
The present invention relates to fixing devices which fix a toner image on a recording medium in image forming apparatuses using, for example, an electrophotographic method.
2. Description of the Related Art
In image forming apparatuses, such as copying machines and printers, which use an electrophotographic method, a photosensitive body (photosensitive drum) formed in the shape of, for example, a drum is uniformly charged and the photosensitive drum is scanned and exposed with light controlled on the basis of image information so that an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image is then turned into a visual image (toner image) with toner. Then, the toner image is directly transferred to a recording medium from the photosensitive drum, or the toner image is primarily transferred to an intermediate transfer medium and secondary transferred to a recording medium from the intermediate transfer medium. Thereafter, the toner image is fixed on the recording medium by a fixing device.
The fixing device used for such image forming apparatuses is composed of, for example, a fixing roller in which a heating source is disposed within a cylindrical core bar, and which is formed such that a heat-resistant elastic layer and a release layer on an outer peripheral surface of the elastic layer are laminated on the core bar; and a pressing roller which is disposed in pressure contact with the fixing roller parallel thereto and formed such that a heat-resistant elastic layer, and a release layer, made of a heat-resistant resin film or a heat-resistant rubber film, on an outer peripheral surface of the elastic layer are laminated on a core bar. Also, a recording medium having an unfixed toner image carried thereon is caused to pass between the fixing roller and the pressing roller so that heating and pressing is performed on the unfixed toner image, thereby fixing the toner image on the recording medium. Such a fixing device is called a two-roller fixing method and it has generally been widely used.
Meanwhile, in such a conventional fixing device using fixing rollers like the two-roller fixing method, the fixing rollers have their own large heat capacity. Therefore, there is a problem in that even when supply of power to the fixing devices is started simultaneously when a power source of the image forming apparatus is turned on, considerable time is required until the fixing rollers rise in temperature from normal temperature to a fixable temperature (warm-up). Because of the characteristics of the fixing rollers that quick start is difficult as such, when the image forming apparatus is in a standby state, it is also necessary to keep the temperature of the fixing rollers always constant to prepare for the start of the image forming operation. Therefore, there is also a problem in that electric power consumption of the fixing device is large.
In order to solve such problems, fixing devices using an endless fixing belt member have been developed instead of the configuration using the fixing roller. Since the fixing belt member has its base member made of a film-like heat-resistant resin or the like, it has advantages in that the heat capacity is small and the warm-up can be performed in a short time, as compared to a roller-shaped member such as the fixing rollers. Moreover, since the quick start is easy, the electric power consumption of an image forming apparatus in a standby state can also be reduced.
As an example of conventional techniques related to the fixing devices using the fixing belt member, the following technique is suggested (for example, see JP-A-2003-223064). According to this technique, the fixing device is configured such that a halogen heater is disposed in an inner space. The fixing device also is composed of a heating film (fixing belt member) rotatably supported by a support member, and a pressing roller member disposed in pressure contact with the heating film to form a fixing nip and to drive the heating film so that the heating film follows the pressing roller member. Infrared rays emitted from the halogen heater are converged on the fixing nip to heat the heating film in the nip part, thereby forming a toner image on a recording medium passing through the fixing nip on demand.
In such a fixing device using the fixing belt member, generally, the fixing belt member is adapted to follow the pressing roller member. However, in such a configuration in which the fixing belt member is caused to follow the pressing roller member, the rotational speed of the fixing belt member may change under the influence of a change in surface velocity of the fixing belt member caused by thermal expansion of the pressing roller member. Further, since the fixing belt member rotates with a frictional force from the pressing roller member, if the sliding resistance between the fixing belt member and a supporting member supporting the fixing belt member increases, slip is caused between the fixing belt member and the pressing roller member. Therefore, the rotational speed of the fixing belt member may be reduced. Accordingly, when a recording medium passes through the fixing nip part, there is a probability that disturbance may be caused in a fixed image on the recording medium or wrinkles are created in the recording medium.
As a method for suppressing such a change in the rotational speed of the fixing belt member, the following technique is also suggested (for example, see JP-A-7-281549). According to this technique, gears are respectively disposed at positions corresponding to an end of a fixing belt member and an end of a pressing roller member, and both the fixing belt member and the pressing roller member are rotated while the gears are caused to mesh with each other, or a recess (or protrusion) and a protrusion (or recess) are respectively disposed at positions corresponding to the end of the fixing belt member and the end of the pressing roller member, and both the fixing belt member and the pressing roller member are rotated while the recess (or protrusion) is caused to engage the protrusion (or recess), thereby suppressing a speed difference between the fixing belt member and the pressing roller member.
Meanwhile, since it is necessary to give a fixed image an appropriate degree of gloss in the fixing device composed of the fixing belt member and the pressing roller member, a predetermined pressure (a nip pressure) is required to be applied to the fixing nip part between the fixing belt member and the pressing roller member. As described above, if gears are allowed to engage each other between the fixing belt member and the pressing roller member between in order to suppress the reduction in rotational speed of the fixing belt member that follows the pressing roller member, a pushing force between the fixing belt member and the pressing roller member is limited by an engaging portion between the gears. Therefore, a problem may occur that it becomes difficult to apply a predetermined nip pressure to the fixing nip part. Accordingly, in order to stabilize the rotational speed of the fixing belt member, it is preferable to employ the configuration in which gear members are mounted on both ends of the fixing belt member, respectively, and the gear members are connected to a driving source to be driven thereby rather than the configuration which the fixing belt member indirectly receives a driving force from the pressing roller member to follow the pressing roller member. In this configuration, even when the pressing roller member thermally expands or the sliding resistance between the pressing roller member and the supporting member increases, the rotational speed of the fixing belt member can be stabilized. Further, since an engaging portion between gears exists between the fixing belt member and the pressing roller member, the pressing roller member can be brought into pressure contact with the fixing belt member with a desired pressure, and a predetermined nip pressure can also be set.
However, in a case in which an object to be rotatingly driven is made of a material having a high strength or rigidity like, for example, a metallic cylindrical body, a groove or a hole is formed in the cylindrical body and a protrusion corresponding the groove or the hole is formed in each gear member. Then, the cylindrical body and the gear member can be combined together by making the groove or the hole in the cylindrical body fit with the protrusion in the gear member. By this combining method, a rotational driving force can be easily transmitted to the cylindrical body without causing breakage, such as deformation or rupture, in the cylindrical body. In contrast, if an object to be rotatingly driven is a thin-walled fixing belt member having a low strength or rigidity, and the above-described general method of fitting the groove or the hole with the protrusion and of then partially combining the fixing belt member and the gear member together at a fitting portion between the groove or hole and the protrusion is used, stress may be concentrated on the fitting portion, which makes it impossible for the fixing belt member to endure the stress at the fitting portion only with its own strength. As a result, breakage such as rupture readily occurs in the fixing belt member. Therefore, there is a significant problem in that not only a driving force cannot be efficiently transmitted, but also breakage of the fixing belt member is finally caused, which makes the functions of the fixing device disabled.